Generally, C/C composite materials are produced by impregnating or mixing long or short carbon fibers such as polyacrylonitrile (PAN) type, pitch type and rayon type with a thermosetting resin such as a phenol resin and a furan resin or a thermoplastic resin such as pitches, followed by heating and then baking them in a non-oxidative atmosphere at a temperature of from 600.degree. to 2500.degree. C.
Various kinds of methods, however, have been known for producing C/C composite materials in which short fibers are used. For instance, a method comprising mixing a solvent and carbon fibers in a mixer, forming into a mat by a screen and then applying a pressure to produce a porous carbon electrode (Japanese Patent Application Laid-Open (KOKAI) Nos. 54-41295(1979) and 57-129814(1982)); a method comprising mixing a solvent and carbon fibers of different length in a molding by ultrasonic vibrations (Japanese Patent Application Laid-Open (KOKAI) No. 58-30537(1983)); a method comprising laminating short fibers into a mat and then impregnating with a resin or pitch and molding (Japanese Patent Application Laid-Open (KOKAI) No. 62-119288(1987)); a method comprising applying beating in a solvent, thereby homogeneously dispersing short fibrous carbon fibers, and then removing the solvent to obtain a carbon fiber mass oriented at random (Japanese Patent Application Laid-Open (KOKAI) No. 62-96364(1987)); and a method comprising supplying a mixture of short fibers and a solvent to a molding having a plurality of solvent drawing ports at the bottom and then drawing out the solvent to form a pre-form (Japanese Patent Application Laid-Open (KOKAI) No. 1-176173(1989)) may be mentioned.
The C/C composite materials produced by the methods described above have been used in recent years as sliding materials of brakes for use in aircrafts or cars or clutches for use in cars, instead of existent metal disc rotors. This is because the C/C composite material is excellent, as compared with the metal disc rotor, in characteristics such as reduced weight and heat resistance.
For using the C/C composite material as the sliding material, it is necessary to control the frictional characteristic or the mechanical strength thereof to appropriate values.
As a method for controlling the frictional characteristic of the C/C composite material, the following methods are known for instance. Namely, a method for controlling the frictional characteristic of the C/C composite material by impregnating with coal tar, pitch, furan resin or the like (Japanese Patent Publication (KOKOKU) No. 60-54270(1985)), a method comprising orienting relatively long fibers (4 to 6 cm) and short fibers (0.015 to 0.3 cm) at random (Japanese Patent Publication (KOKOKU) No. 1-59459(1989)) and a method comprising cutting a carbon fiber reinforced thermosetting resin composite material in which carbon fibers are oriented so as to transverse a plane perpendicular to an axis of a cylinder along a direction in perpendicular to the axis of the cylinder, baking and densifying it to orient the carbon fibers such that they have a certain angle relative to a sliding surface (Japanese Patent Publication (KOKOKU) No. 3-78498(1991)) may be mentioned.
As a method for controlling the mechanical strength of the C/C composite material, the following methods have been known for instance. That is, a method comprising using short fiber reinforced type C/C composite material of excellent coefficient of friction only to the sliding portion at the surface and using a two-dimensional fabric (so-called woven fabric) to the core portion as a reinforcing material for compensating the mechanical strength of the short fiber reinforced type C/C composite material (Japanese Patent Application Laid-Open (KOKAI) No. 2-266239(1987)), and a method comprising forming a sliding layer at the surface as a "fine" texture comprising segments in which short fibrous carbon fibers are distributed at random and having a mean pore size of less than 50 .mu.m, while forming a structural layer at the inside as a "coarse" texture comprising a mesh of short fibrous carbon fibers and having a mean pore size of more than 100 .mu.m (EP 0459916 A1) may be mentioned.
Further, for a practical use as a sliding material, it is necessary for wear resistance under high load condition such as high energy level and high pressure. For this purpose, it has been conducted to increase density (reduction of porosity) and increase strength by repeating a densifying treatment. As such a method, it has been known, for example, a method of impregnating with a thermosetting resin such as a phenol resin or a furan resin or a thermoplastic resin such as pitches, or a CVD process in which a gaseous organic material such as methane, propane and dichloroethylene, is used as a raw material.
However, in the above-mentioned proposals, there have been still present a lot of problems to be solved regarding not only the producing method of the C/C composite material but also for the use as the sliding material.
For instance, the producing method of the C/C composite material involves the following problems. In the method as described in Japanese Patent Application Laid-Open (KOKAI) No. 62-119288(1987), since mats formed from short fibers (non-woven fabrics) are laminated and the laminate is impregnated with a resin or pitch, no sufficient resin or pitch is impregnated into each of non-woven fabrics, making it difficult to mold a C/C composite material of large thickness. In addition, large voids are liable to be formed between each of carbon fibers or between each of laminated carbon fiber layers, and accordingly this tends to result in a C/C composite material of poor strength. Further, the method as described in each of the Japanese Patent Application Laid-Open (KOKAI) Nos. 62-96364(1987) and 1-176273(1989) involves a drawback of tending to cause an uneven distribution of carbon fibers upon preparing a thick pre-form. Further, since fibers are oriented in a pseudo three dimensional manner in other methods, it tends to result in uneven distribution of carbon fibers, local voids, and non-uniformity in view of the strength, so that there is a problem of failing to obtain a uniform characteristic upon using as a frictional material or sliding material.
In order to overcome the foregoing problems, the present inventors have previously proposed a method comprising dispersing short fibrous carbon fibers and preparing a sheet in a two-dimensional random orientation in Japanese Patent Application Laid-Open (KOKAI) No. 3-140211(1991), but still it can not be said satisfactory in view of weight reduction and strength of the C/C composite material.
Further, in particular, with respect to the impregnating treatment in the method for producing the C/C composite material, there are the following problems. Namely, in the case of using a thermosetting resin such as a phenol resin or furan resin, since pores are liable to be closed to leave pores at the inside during repeating of impregnation, curing and baking steps, it is difficult to reduce the porosity. Further, in the case of a CVD process using a gaseous organic material such as methane, propane and dichloroethylene as a raw material, adhesion with carbon fibers is poor, resulting in remarkable consumption by abrasion upon application to a brake sliding portion.
In the case of using a thermoplastic resin such as pitches, those having high softening point are generally used in order to obtain high carbonization yield. However, since a pitch having high softening point is poor in impregnating characteristic and requires high temperature treatment, excessively large facility is required for producing C/C composite materials and handling becomes troublesome. That is, each of the above-mentioned methods has respective merits and demerits and is not satisfactory.
Regarding the control for the characteristic as the sliding material, there are the following problems.
The method described in Japanese Patent Publication (KOKOKU) No. 60-51270(1985) notes just a matrix and does not mention essential orientation of carbon fibers. The method described in Japanese Patent Publication (KOKOKU) No. 1-59459(1989) needs the mixing of two different kinds of carbon fibers and any particular effect of carbon fiber length is not shown. Further, the method as described in Japanese Patent Publication (KOKOKU) No. 3-78498(1991) involves a problem that a disc in which carbon fibers are oriented so as to transverse a plane in perpendicular to the axis of a cylinder, has to be cut crosswise for production, and therefore producing steps are complicate.
Further, the above-mentioned proposal shows no concrete descriptions regarding control for the frictional characteristic, and merely shows general tendency. On the other hand, the proposal in Japanese Patent Application Laid-Open (KOKAI) No. 62-266239(1987) regarding control for the mechanical strength is expensive because woven fabrics are used as the structural material for the central portion, and it only suggests reinforcement without considering a balance of the strength. Further, the proposal of EP 0459916 A1 refers only to the abrasion amount and reinforcement for the strength.
As described above, the proposals made so far mentions nothing about the control of the frictional characteristic and the balance of the strength.
The present inventors have made various studies for overcoming the foregoing problems and obtained the following findings.
(1) In the case of dispersing a short fibrous bundle of carbon fibers to prepare a sheet in which fibers are in a two-dimensional random orientation, the frictional characteristic and the mechanical characteristic of the C/C composite material change depending on the degree of dispersion for the bundle of carbon fibers. For instance, the coefficient of friction becomes low as the dispersion degree of the bundle of carbon fibers becomes high, and it becomes high as the dispersion degree becomes low. The compression strength becomes high as the dispersion degree of the bundle of carbon fibers becomes high. The tensile strength, bending strength and impact strength become high as the dispersion degree of the bundle of carbon fibers becomes low.
(2) Contrary to the common knowledge in the prior art, a C/C composite material produced from carbon fibers which are sizing-free has remarkably high wear resistance.
(3) A C/C composite material having a final porosity of not more than 20% by volume, preferably not more than 10% by volume obtained by a particular method is firm in adhesion of carbon fibers and a matrix to cause less fracture at micro level, and exhibits excellent wear resistance.